1. Field of the Invention
The present invention is directed to a system and method for improved mobile telephone location, particularly, to a system and method for identifying an elliptical area about an estimated mobile telephone location within a telecommunications system, the elliptical area being an optimum area for locating the actual mobile terminal.
2. Background and Objects of the Present Invention
The evolution of wireless communication over the past century, since Guglielmo Marconi's 1897 demonstration of radio's ability to provide continuous contact with ships sailing the English Channel, has been remarkable. Since Marconi's discovery, new wireline and wireless communication methods, services and standards have been adopted by people throughout the world. This evolution has been accelerating, particularly over the last ten years, during which time the mobile radio communications industry has grown by orders of magnitude, fueled by numerous technological advances that have made portable radio equipment smaller, cheaper and more reliable. The exponential growth of mobile telephony will continue in the coming decades as well, as this wireless network interacts with and eventually overtakes the existing wireline networks.
Pursuant to a recent Federal Communications Commission (FCC) Ruling and Order, cellular phone service providers within the United States must provide by October, 2001 the capability to locate the position of a cellular phone making an emergency (911) call within the provider's system to within 125 meters with at least 67% probability. In other words, the FCC requires Automatic Location Information (ALI) with a radial error no greater than 125 meters for the location of a cellular phone call with 67% probability. A Public Safety Answering Point (PSAP) would then dispatch response units to the center of this circle and instruct them to search out to a certain radius from the center in the event that the emergency is not immediately discernable or visibility is limited (fog, rain, intervening obstacles, etc.).
It should be understood that an optimum search area should include all regions where the chances of finding the caller are good and exclude regions of poor probability. All positioning algorithms known to be currently under consideration involve locating the phone with respect to known locations of base stations or satellites. While the FCC mandate is based upon a circular area, however, all the known preferred techniques predict a more elliptical, non-circular area. In other words, the errors in the known techniques are elliptical to the lowest order in the calculation, i.e., a contour of constant probability density for locating the phone is an ellipse.
Nonetheless, pursuant to the FCC directive, since the equipment must satisfy a circular requirement, a circular contour presented to the user might at first glance seem to be the appropriate solution. As is understood in the mathematical arts, when the coverage probability of an ellipse is known, to then calculate the probability bounded by a perfect circle, a non-trivial numerical integration is required. Alternatively, an approximation may be used. In view of the actual elliptical nature of the densities, however, this integration calculation is wasteful and unnecessary and increases the time before the display is ready for the PSAPs. Worse still, if the emergency is not immediately visible to the response units due to intervening buildings or terrain, the PSAP must provide directions as to where to search. In this case, the response units might have to search the entire circular area.
In many cases, however, particularly along the fringes of the coverage area, a circular region is such a poor approximation of the true error ellipse that emergency units may spend considerable vital time searching in the wrong places and not enough time searching in high probability density areas, which as discussed are actually elliptical in nature. Further, it has been found that a circle so formed and centered about the indicated position has as much as four or more times the area of the elliptical region, almost all of the extra area being unproductive.
It is, therefore, an object of the present invention to provide an improved system and method for indicating an optimal search area about the estimated location of a cellular phone caller within a telecommunications system.
It is a further object of the present invention that the improved system and method facilitate such search area definition pursuant to the FCC mandate, providing PSAPs with a tool to focus search, rescue and emergency efforts to a region having the highest probability of success, reducing the time required to locate the cellular phone caller.